1. Field of the Invention
This invention relates to a cement dispersant capable of preventing slump loss. More particularly it relates to an agent which is incorporated in a cement composition such as cement paste, cement mortar or concrete for the purpose of heightening the flowability thereof and, at the same time, preventing the flowability from deteriorating with elapse of time (hereinafter referred to as "slump loss") thereby enhancing the workability thereof.
2. Description of the Prior Art
The cement composition such as cement paste, cement mortar or concrete is caused as by the hydration of cement with water to incur loss of consistency and decline of workability with elapse of time after the mixing of ingredients thereof. This phenomenon is generally called "slump loss".
The slump loss in the cement composition, e.g. in freshly mixed concrete, entails various hindrances such as limitation of time allowed for transportation, degeneration of quality and impairment of workability due to protracted waiting time at the site of placement, and loss of durability due to cold joint. At a factory for the manufacture of secondary products of concrete, the slump loss can cause sharp rise of hydraulic pressure for conveyance or full closure of a pump when the conveyance of cement composition is temporarily suspended during a lunch time or on account of mechanical trouble and it is restarted later on. In the case of a mold to be used for the formation of a shaped article of concrete, the slump loss can cause the problem of incomplete filling, for example, when the work for compaction is delayed by some reason or other after the placement of cement composition has been completed. The slump loss in the cement composition, therefore, constitutes itself an important problem to be solved for the control of quality and for the improvement of workability of the cement composition at plants for production of ready-mixed concrete and plants for manufacture of secondary products.
Heretofore, as measures for the prevention of slump loss, the following methods have been known to the art.
(A) A method which relies on an increase in the unit amount of water in concrete.
(B) A method which resorts to late addition of a cement dispersant.
(C) A method which resorts to piecemeal addition of a cement dispersant.
(D) A method which relies on addition of a setting retardant, optionally in combination with a cement dispersant.
The method of (A) mentioned above comprises increasing the unit amount of water with a due allowance for the slump loss estimated to develop by the time of actual placement of the cement composition. Though this method attains improvement in workability, it entails not only a disadvantage in terms of quality that the set product suffers from inferior durability because of loss of strength and formation of cracks due to dry shrinkage but also an economic disadvantage that the unit consumption of cement must be inevitably increased.
The method of (B) is meant as a measure for temporary improvement in flowability. The cement dispersant, even after it has fulfilled the purpose of retaining the slump intact, it locally remains in the cement composition and entails such adverse effects as local occurrence of bleeding and consequent loss of strength.
The method of (C) comprises repeating the addition of a cement dispersant each time the slump loss manifests itself. It is incapable of thoroughly preventing the slump loss. Moreover, it inevitably entails an operational and economic disadvantage of repeating addition.
The method of (D) is aimed at retaining the flowability intact by using a retardant such as an oxycarboxylate, a lignin sulfonate, dextrin, or humic acid either alone or in combination with a cement dispersant thereby retarding the time for setting the cement composition. This method has a large possibility of incurring such drawbacks as loss of strength and insufficient setting when the additives are used excessively.
Thus, these methods invariably have demerits of their own and do not prove to be fully practicable.
An object of this invention, therefore, is to provide a novel cement dispersant, a cement composition, and a method for dispersion of cement in the cement composition.
Another object of this invention is to provide a cement dispersant for heightening flowability of cement composition and preventing the cement composition from slump loss, a cement composition incorporating the cement dispersant, and a method for dispersion of cement in the cement composition.